Various kinds of fibers for optical communication are known such as the quartz type, multicomponent glass type and plastic type fibers. Of those, the quartz type optical fibers are most widely used from the standpoints of their low loss of optical transmission, transmitting capacity, heat resistance, weatherability, and high reliability. However, the quartz type optical fibers are disadvantageous in that the fibers will grate fine surface defects and the properties of the fibers will change with the lapse of time. In order to overcome those problems, the surface of quartz-type optical fibers is primarily coated with a silicone resin and then a finish coating is applied thereon. The reasons why a silicone resin has been selected as the primary coating are mainly as follows: (1) the silicone resin has a reinforcing effect against the quartz fibers so that the reinforced fibers can exhibit a strength substantially the same as the theoretical value; and (2) the silicone resin shows stable physical properties which vary little over a wide temperature range, and particularly the Young's modulus changes little at low temperatures, and hence the silicone resin is useful for stress relaxation, so that the increase in transmission loss due to microbending and the adverse effect of scattering on noise levels can be minimized. As a silicone resin for this use, an addition reaction-curable liquid silicone has so far been used, which cures upon heating. However, rationalization of the coating process has reached its limit in view of the low curing rate and production efficiency, even where a high-temperature heating chamber is used. Further, use of this addition reaction-curable silicone has newly caused a serious problem that an organopolysiloxane used as a crosslinking agent, which contains hydrogen atoms bonded to silicone atoms (i.e., which has Si-H bonds) generates hydrogen gas, and the gas adversely affects the transmission characteristics of the quartz glass fibers.
Under those circumstances, attention is now focused on ultraviolet-curable silicone resins. JP-A-58-187902 discloses a coated optical fiber prepared by applying on an optical fiber a composition comprising a polysiloxane having a carbon-carbon double bond in the molecule, and a polymerization initiator and/or an organic compound having an SH group, and then curing the applied composition by ultraviolet irradiation. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.) JP-A-58-204845 discloses a process for preparing a glass fiber for optical transmission, which process includes use of a composition comprising the above-described polysiloxane, a photosensitizer, and an SH group-containing organopolysiloxane. However, the two compositions described above have disadvantages of an offensive odor and poor stability.
Compositions similar to the above ones are described in JP-A-59-35044 and JP-A-59-88344. There is a description therein that an antioxidant may be incorporated as an optional component from the standpoint of the insufficient stability of the above-described compositions. Although improved to some extent by the optional component, the stability of those compositions is still insufficient when they are used for coating optical fibers.
JP-A-60-110752 discloses a composition comprising an alkenyl group-containing linear diorganopolysiloxane, a mercapto-functional organopolysiloxane, a photosensitizer, a viscosity stabilizer and a reinforcing agent. This composition shows an improved curing rate and gives a cured resin having improved strength, but the stability of the composition and the heat resistance of the cured resin are still insufficient.
JP-A-62-79265 discloses a composition comprising a mercaptosiloxane unit-containing organopolysiloxane, an unsaturated group-containing organopolysiloxane, benzophenone, and a second initiator. However, this composition is not suitable for application to optical fibers since it is difficult to obtain transparency of the composition.
Further, JP-A-62-161856 discloses a composition comprising an alkenyl group-containing organopolysiloxane, a mercaptoalkyl group-containing silicone resin, a mercaptoalkyl group-containing diorganopolysiloxane, and a photosensitizer. This composition gives a cured resin having improved strength and low-temperature properties, but the heat resistance and weatherability of the cured resin are insufficient to some extent.
As described above, the conventional compositions have the following disadvantages: the storage stability of the compositions before curing is poor; the photosensitivities of compositions are so low that they cure only when the radiation energy is more than several hundred mJ/cm.sup.2 ; the high-speed wire-drawing properties of the compositions are poor; and cured resins coated on fibers deteriorate due to ultraviolet rays, causing an oily substance to bleed out from the surfaces, whereby the application of nylon coatings on the resulting surfaces to form nylon jackets cannot be performed uniformly. Further, since the heat resistance of the compositions which have so far been proposed is not so good as compared with other general-purpose organic materials, the heat life of optical fibers has become a serious problem.